Storage

Why does my iphone storage say that messages take up less space than top conversations?

Under general/iphone storage/messages, it says documents/data 15.9gb. Directly under it, it says Top conversations 25.13.gb. Most importantly, my messages ARE setup to save to the icloud.. so also, why are they taking up any space at all, let alone somewhere between 15-30gb.... (this is all under "iphone storage" so it's supposed to be telling me what's LOCAL to the iphone, not the icloud storage)